In Game of Thrones, all the Dothraki warriors have long hair and braids. If a Dothraki loses a fight, he cuts off a braid, so the longer the braid, the stronger the warrior. Khaleesi follows this tradition and she starts to add braids from the moment she becomes the queen of the khalasar. The more victories she has, the more braids appear on her head: by the end of the show, her hair is more complicated than the last season of the show.
Internet users even joked about this, “It is great that the showrunners didn’t put bells in her braids like they did in the books. Because then, her dragon flights would sound like a Christmas sleigh.”